4 keys: soybean disease management

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There are four components of successful late-season soybean disease management.

“Producers should use as many of these components as possible,” said Boyd Padgett, LSU AgCenter plant pathologist at the recent Tri-State Soybean Forum in Oak Grove, La. “These components are like gears in a machine working together.”

• Identify the disease properly: Proper disease identification is the first step to an effective management strategy. This is critical because “knowing what’s affecting your crop will determine what treatments you implement. This can save producers time and money.”

Identifying the malady will not only help in treating the problem, but can also help keep it from spreading. This is particularly true of soil-borne issues like nematodes and red crown rot. Dealing with soil-borne diseases while “using plows and the like in an infected field can result in field-to-field spread if they aren’t properly cleaned between fields. If a producer has 10 fields of soybean and only one has nematodes, producers should make every effort to keep the problem (in the single location). Once a field has a soil-borne disease problem, it’s like trying to take sugar out of sweet tea — it’s probably not coming out.”

• Utilize resistant varieties: Growers should pick varieties based on tests closest to their operation.

“The reaction of those varieties in different locations will show variances for a number of reasons. Agronomic characteristics are one thing — some varieties may yield better in lighter soils, some in heavier. Also, it seems that there may be a difference in the disease-causing organisms between locations and how they affect the crop.”

Padgett pointed to a study of four soybean varieties. All were tested in both Jeff Davis Parish and in Macon Ridge Research Station fields. The study shows the same variety can react differently to a particular disease depending on location.

“(Variety A) rated a 7 in Jeff Davis and an 8.3 at the Macon Ridge Research Station — it was affected equally at both. However, (variety B) only got a rating of 2 at Jeff Davis and pulled an 8.3 at the Macon Ridge station. (Variety C) rated a 4 and 7.8, respectively, while (variety D) rated a 3 and 8.5.”

That gap in ratings could be due to a number of things. One possibility, suggested Padgett, is diversity in the cercospora blight pathogen population.

“Just like in humans: some folks never get sick and others can look at a picture of a germ in a magazine and catch a cold. Growers should find the variety that’s less likely to catch a cold in their neck of the woods. There are no guarantees, but if I’m growing beans in Franklin Parish, it makes sense for me to be checking the variety research from the Macon Ridge station data instead of something from the rice station.”

• Cultural practices: Producers should know how to manage for disease in certain cropping situations. For instance, with cotton seedling disease, “they can utilize planting dates and go forward only when the weather is right. The same can be true with some soybean diseases, which overwinter on plant debris. With increased no-till practices in the Mid-South, that can be a concern.”

The red crown rot fungus prefers cool, wet soils. “So, in a field where a producer knows there’s a problem with that, soybeans should be planted later in well-drained fields. That certainly lessens the risk.”

• Fungicides: Timely applications are extremely important with fungicides.

“Most growers’ fungicide program foundations will contain a strobilurin. I’m not trying to be an advocate for a certain product, but a strobilurin is a given. Then, based on what the crop is faced with, growers will add to that — maybe something for soybean rust or cercospora blight.”

As for residual activity, “there’s typically anywhere from two to four weeks. The length of time depends on a number of things including the specific disease and weather. Also, the growth stage of the plant plays a role — if it’s young and putting foliage on, three days after application there will be newly emerged leaves that are unprotected.”

What about Asian soybean rust? As of mid-January, sporulating soybean rust had already been found in Louisiana kudzu. “I believe this is the earliest it’s been found in the state and was located in Washington, East Baton Rouge, Tangipahoa, Iberville and St. Mary parishes. The fact that it can be established so early proves the organism is in the area. Rust has also already been found in Alabama and Texas.”

Even though Mid-South soybean crop yields have yet to be hammered with rust, “it continues to be worrisome. Growers may be tired of hearing it, but the potential is there. If a stable, favorable weather pattern for the disease lingers, it could continue to spread.”

Padgett believes that over time — “and I hope it’s many, many years” — rust could become established in the Mid-South “and we’ll have to deal with it regularly. Remember, at one time, cercospora blight wasn’t a big deal. When I was in graduate school, it was only a south Louisiana problem. Now, it’s one of the top diseases, if not the number one soybean disease, in the Mid-South. Hopefully, rust will never get established to that point.”